Thu, Jan 21, 2016, 2:00 pm to 3:00 pm
dynamics of electrons and holes in semiconductors. The key to optical control is the strong spin-orbit selection rules that govern absorption near the bandgap, which permit photo-generation and detection of specific spin states (and in certain special cases, specific valley states). This talk will describe how we have applied state-of-the-art optical techniques to explore new physics in two different classes of two-dimensional semiconductor systems: i) the dynamics of the “persistent spin helix” of electrons in GaAs quantum wells, and ii) spin and valley dynamics of electrons in atomically thin transition-metal dichalcogenides (e.g. MoS2). First, we describe a new experimental technique – Doppler spin velocimetry [1,2] – that we developed to study the temporal and spatial dynamics of the “persistent spin helix” [3,4], which is a spin texture that can form in n-GaAs quantum wells. Using this method, which can resolve nanometer-scale displacements of the electron spin polarization on subpicosecond time scales, we found that the spin helix velocity changes sign as a function of wave vector and is zero at the wave vector that yields the largest spin lifetime , and the velocity of spin polarization packets becomes equal to the drift velocity of the high-mobility electron gas in the limit of small spin helix amplitude . More recently, we have directly measured the coupled spin and valley dynamics of resident electrons in the two-dimensional “Dirac semiconductor” molybdenum disulfide (MoS2) using optical Kerr-rotation spectroscopy . These measurements revealed very long spin lifetimes of resident electrons exceeding 3ns at 5K (i.e., orders of magnitude longer than the typical exciton lifetimes that have been primarily studies to date). In contrast with conventional III-V or II-VI semiconductors, spin relaxation accelerates rapidly in small transverse magnetic fields. This suggests a novel mechanism of electron spin dephasing in monolayer transition-metal dichalcogenides, driven by rapidly-fluctuating internal spin-orbit fields due to fast intervalley scattering .  Luyi Yang et al., Phys. Rev. Lett. 106, 247401 (2011).  Luyi Yang et al., Nature Physics 8, 153-157 (2012).  B. Andrei Bernevig, J. Orenstein, and Shou-Cheng Zhang, Phys. Rev. Lett. 97, 236601 (2006). J.D. Koralek et al., Nature 458, 610-613 (2009).  Luyi Yang et al., Nature Physics 11, 830-834 (2015).